One of the coding schemes for an audio signal is High Efficiency-Advanced Audio Coding (HE-AAC). In HE-AAC, low-frequency components of an audio signal are encoded with AAC encoding, and high-frequency components are encoded with spectral band replication (SBR) encoding, thereby improving the coding efficiency.
An exemplary encoding apparatus of the related art will be described which encodes an audio signal with HE-AAC. FIG. 23 is a diagram illustrating a configuration of an encoding apparatus 50 of the related art. As illustrated in FIG. 23, the encoding apparatus 50 includes a downsampler 10, an AAC encoder 20, an SBR encoder 30, and a multiplexer 40.
The downsampler 10 is a processor that performs downsampling on an audio signal. The downsampler 10 outputs the audio signal having a low-frequency component obtained through the downsampling, to the ACC encoder 20.
The ACC encoder 20 is a processor that applies ACC to the audio signal having the low-frequency component so as to encode the audio signal having the low-frequency component. The ACC encoder 20 outputs the encoded audio signal having the low-frequency component to the multiplexer 40.
The SBR encoder 30 is a processor that encodes the high-frequency component of the audio signal. The SBR encoder 30 outputs the encoded high-frequency component of the audio signal to the multiplexer 40. The SBR encoder 30 controls quantization of the audio signal in such a manner that the time resolution is set to high when the audio signal has a transient, or that the frequency resolution is set to high when the audio signal is stationary. The state in which an audio signal has a transient means that, for example, the audio signal includes an abrupt amplitude change.
The multiplexer 40 is a processor that multiplexes the encoded audio signal having the low-frequency component and the encoded audio signal having the high-frequency component and that outputs the multiplexed audio signal to an external apparatus.
Now, an example of the SBR encoder 30 illustrated in FIG. 23 will be described. FIG. 24 is a diagram illustrating a configuration of the SBR encoder 30. As illustrated in FIG. 24, the SBR encoder 30 includes an analysis filter bank 31, a transient detector 32, a grid information generator 33, a spectrum estimator 34, an additional information determiner 35, a quantizer 36, and a multiplexer 37.
The analysis filter bank 31 is a processor that transforms an audio signal into a time-frequency spectrum. The analysis filter bank 31 outputs the audio signal subjected to a time-frequency-spectrum transformation to the transient detector 32, the spectrum estimator 34, and the additional information determiner 35.
The transient detector 32 is a processor that analyzes the audio signal and that detects a state in which the audio signal has a transient. The transient detector 32 outputs the detection result to the grid information generator 33.
FIG. 25 is a diagram for explaining a process performed by the transient detector 32. As illustrated in FIG. 25, the transient detector 32 sets a detection range 60, and divides the detection range 60 into 16 sections. The detection range 60 is set so as to start in a frame 1A and end in a frame 2A. The frame 1A is a target frame to be subjected to SBR encoding, and the frame 2A is subsequent to the frame 1A. The transient detector 32 analyzes the detection range 60 and detects a section in which a signal having an abrupt amplitude change is included. Then, the transient detector 32 outputs the presence/absence of a transient and the position of the transient signal to the grid information generator 33. The transient detector 32 determines presence/absence of a transient for each of the frames.
The grid information generator 33 is a processor that controls the quantizer 36 so that the time resolution is set to high when the audio signal has a transient, and the frequency resolution is set to high when the audio signal is stationary.
The spectrum estimator 34 is a processor that outputs, to the quantizer 36, supplementary information used for replicating the high-frequency component from the low-frequency component. The additional information determiner 35 is a processor that outputs, to the quantizer 36 and the multiplexer 37, additional information representing the high-frequency component of the audio signal.
The quantizer 36 is a processor that encodes the high-frequency component with the time resolution and the frequency resolution which are determined under the control of the grid information generator 33. The quantizer 36 outputs the encoded high-frequency component of the audio signal to the multiplexer 37.
The multiplexer 37 is a processor that multiplexes the encoded audio signal having the high-frequency component, which is output from the quantizer 36, and the additional information, and outputs the multiplexed information.
However, in the related art described above, there is a problem in that the implementation scale and the processing load are large.
As illustrated in FIG. 24, since the transient detector 32 is implemented to detect a transient in an audio signal, the SBR encoder 30 has a large implementation scale. In addition, as illustrated in FIG. 25, since the detection of a transient is performed for each of frames, the transient detector 32 has a heavy processing load.
Regarding the related art, see Japanese Laid-open Patent Publication No. 2008-129541.
In addition, regarding the related art, see Suzuki, Masanao, Ota, Yasuji, and Ito, Takashi, “Wansegu Housou Muke Audio Fugouka Gijutsu (Audio Coding Algorithm for One-Segment Broadcasting),” FUJITSU.58, 2, pp. 162-167, March 2007.